Eclipsing Binaries Found by the EREBOS Project: Gaia DR2 6097540197980557440 -- A Deeply Eclipsing sdB+dM System

Abstract

We present time-series spectroscopy and photometry of Gaia DR2 6097540197980557440, a new deeply-eclipsing hot subdwarf B (sdB) + M dwarf (dM) binary. We discovered this object during the course of the Eclipsing Reflection Effect Binaries from Optical Surveys (EREBOS) project, which aims to find new eclipsing sdB+dM binaries (HW Vir systems) and increase the small sample of studied systems. In addition to the primary eclipse, which is in excess of $\sim$5 magnitudes in the optical, the light curve also shows features typical for other HW Vir binaries such as a secondary eclipse and strong reflection effect from the irradiated, cool companion. The orbital period is 0.127037 d ($\sim$3 hr), falling right at the peak of the orbital period distribution of known HW Vir systems. Analysis of our time-series spectroscopy yields a radial velocity semi-amplitude of $K_{\mathrm{sdB}}=100.0\pm 2.0{\mathrm{km}\mathrm{s}}^{-1}$, which is amongst the fastest line-of-sight velocities found to date for an HW Vir binary. State-of-the-art atmospheric models that account for deviations from local thermodynamic equilibrium are used to determine the atmospheric parameters of the sdB. Although we cannot claim a unique light curve modeling solution, the best-fitting model has an sdB mass of $M_{\mathrm{sdB}}=0.47\pm 0.03M_{\odot }$ and a companion mass of $M_{\mathrm{dM}}=0.18\pm 0.01M_{\odot }$. The radius of the companion appears to be inflated relative to theoretical mass-radius relationships, consistent with other known HW Vir binaries. Additionally, the M dwarf is one of the most massive found to date amongst this type of binary.

arXiv comment15 pages, 9 figures, accepted for publication by ApJ

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